The polymer is also packed with small fragments of genetic material designed to mimic bacterial DNA. These fragments signal to the dendritic cells that a foreign invader is present. Also present are ground-up pieces of the patient's tumor, which show the cells what to attack. The dendritic cells pick up these molecules as they move through the scaffold. The cells then travel to the lymph nodes, where they introduce the target molecules and generate an immune response. "When the implant is in the body, the immune system sees it as dangerous material and attacks it," says Tarek Fahmy, a bioengineer at Yale University who was not involved in the research.

In mice with established melanoma tumors, the vaccine significantly slowed the growth of the tumors and increased animals' survival time. In addition, tumors completely disappeared in 20 to 50 percent of animals given two vaccinations, depending on how long the tumors had been growing. Researchers say this is significant, given that most cancer vaccines considered to be effective in rodents have been shown to prevent formation of tumors rather than to diminish established tumors. However, it's difficult to compare different rodent models of cancer, which can vary widely.

The implant's effectiveness may lie in the immune response that it triggers, says Mooney. It appears to generate the formation of different types of dendritic cells, which may make the immune response more potent. It also appears to dampen a part of the immune system that typically neutralizes the response once it's been activated--maintaining an activated immune system might be important in preventing tumors from recurring. "That is very novel and extremely important for cancer immunotherapy," says Fahmy.

As is often the case with new cancer treatments, it's difficult to predict how well the findings will translate to humans. A number of cancer vaccines have shown success in animal models and then failed in human clinical trials.